Why can't you measure the temperature of multiphase flow's?

I've read from many sources that thermocouples or any other type of thermal transducer can't accurately measure the temperature of multiphase flows but none of them explain why. For instance, a mixture of air and ethanol going through a fuel line at 80 C. Why is it that if you stick a thermocouple in the fuel line to measure temperature, it won't give you an accurate result? At 80C there shouldn't be much of an effect of radiative heat transfer and given a steady flow and temperature there should be no issues with changes in convective heat transfer. So what gives, why can't you do this?